The switched mode power supply (SMPS) is a well-known type of power converter having a diverse range of applications by virtue of its small size and weight and high efficiency. For example, SMPSs are widely used in personal computers and portable electronic devices such as cell phones. An SMPS achieves these advantages by switching a switching element such as a power MOSFET at a high frequency (usually tens to hundreds of kHz), with the frequency, or duty cycle, of the switching being adjusted to convert an input voltage to a desired output voltage.
An SMPS may take the form of a rectifier (AC/DC converter), a DC/DC converter, a frequency changer (AC/AC) or an inverter (DC/AC).
SMPSs are characterised by a transformer with a primary side and a secondary side that respectively define the input side and output side of the SMPS. The transformer provides isolation between the input and output sides and it is important that this isolation between the input and output of the SMPS is maintained.
A pulse width modulator (PWM) generates a duty cycle signal. The duty cycle signal is used to switch at least one transistor so as to apply the input voltage, at the duty cycle, at the inputs to the primary side of the transformer.
The input voltage at the primary side causes a voltage to be induced at the secondary side of the transformer that, for SMPS designs with a DC output, is then rectified to provide the output voltage.
With DC/DC converters and AC/DC converters in particular, voltage control is applied on the secondary side of the SMPS.
Voltage control is required for meeting the increasing requirements of regulation and dynamic performance (such as monotonic start-up, pre-bias immunity, accurate load regulation and remote voltage sensing).
Generally, it is preferred to use the secondary side alone for performing the control of the output voltage as this gives the lowest distortion of the desired ramp-up curve as well as accurate start-up performance. This can be achieved by providing a voltage regulator on the secondary side to perform voltage control.
Known systems use an additional power supply to provide a power source to the secondary side voltage regulator so that the voltage regulator can start the converter. The additional power supply is usually provided by a second transformer, within the SMPS, that transfers power from the primary side across the isolation barrier. Alternatively, a separate and independent power supply unit may be provided on the secondary side.
However, the above approaches of providing an additional power supply suffer from a number of problems. In particular, they increase the cost of the SMPS, it is more difficult to achieve higher insulation voltages, and the size of the SMPS is increased.
In view of the above problems, known systems have powered a secondary side voltage regulator with power from the primary side by transferring power across the same transformer used to transfer power to the output of the SMPS. In this way, an additional transformer is not required for transferring power across the isolation barrier.
However, with known systems that adopt the above approach, power supplied to switch on the voltage regulator is also supplied to the output of the SMPS. This causes problems when the SMPS is switched on since, to ensure a reliable start-up, it is necessary for the voltage regulator on the secondary side to already be switched on and controlling the output voltage when the output voltage is changing to its desired value. However, when such systems transfer power to switch on the voltage regulator, any excess power appears at the SMPS output. This can cause obstacles such as glitches, plateaus and distortion of the desired start-up ramp figure (i.e. the excess power causes the start-up to be non-monotonic).
Moreover, the required power to switch on the voltage regulator varies with the operating conditions (e.g. temperature). Known systems that use the same transformer to transfer power to both the output and to switch on the voltage regulator therefore need to transmit sufficient power for the voltage regulator to start under the most power demanding conditions. The problems caused by transmitting excess power cannot therefore be avoided.